Conventional approaches for preparing inorganic microstructures such as planar optical devices (e.g., optical waveguides) involve numerous time-consuming process steps and complex, expensive capital equipment. The most common methods of making waveguides consist of depositing the waveguide layers by plasma-enhanced chemical vapor deposition (PECVD), or flame hydrolysis deposition (FHD). A typical process for waveguide fabrication begins with deposition of a low-index optical cladding layer on an optically-flat substrate. This layer can be annealed to consolidate it (FHD), or densify and stabilize it (PECVD). Next a higher-index optical core layer is deposited on top of the lower cladding layer; it is also typically annealed. To produce useful devices, this core layer must be patterned into ridges that will form the waveguides. The patterning is typically done by first depositing a thin layer of a slowly etching material on the core to act as an etch barrier. The etch barrier is then coated with photoresist, which must be subsequently soft baked, imaged via exposure to ultraviolet light in a mask aligner, chemically developed and hard baked. The photoresist image is transferred to the etch barrier via a first etch step, then transferred to the core material via a directional etching process such as reactive ion etching. Finally, the waveguide ridges are covered with a lower-index upper optical cladding layer, which can be annealed after deposition. Although excellent results can be achieved using vapor deposition and reactive ion etching techniques, the deposition and patterning processes are complex and relatively slow (the deposition and etching steps can each require several hours), and the equipment is expensive (the chemical vapor deposition (CVD) and reactive ion etching(RIE) units alone each cost several hundreds of thousands of dollars). Thus, it is difficult to manufacture very low cost devices, especially in low volume.
Sol-gel methods for waveguide fabrication potentially allow the deposition of films for waveguide fabrication without the use of FHD or PECVD, using relatively simple and inexpensive spin or dip coating equipment. Furthermore, these materials can potentially be photosensitized and directly patterned using traditional lithography, thereby eliminating many process steps and the need for RIE equipment. Unfortunately, sol-gel fabrication of high-quality layers suitable for waveguides has proven extremely challenging. This is because of the difficulty of producing uniform, crack free films having appropriate thicknesses for waveguide structures (typically on the order of 6–10 microns). This difficulty is largely due to shrinkage that occurs as sol-gel films are dried or sintered. Consequently, using sol-gel processing for fabrication of inorganic devices with dimensions suitable for good mode matching to single mode or multi-mode optical fiber can be difficult.